Superalloys are nickel, cobalt, or iron base materials, and have useful mechanical properties at temperatures on the order of 1,000.degree. F. and above. Because of their desirable properties, superalloys have found numerous applications in gas turbine engines. In general, components for gas turbine engines are either cast, fabricated by powder metallurgy techniques, or are fabricated and machined from thermo-mechanically worked product forms such as forgings, plate, and sheet. Thermo-mechanically worked products usually have a finer grain size and more homogeneous microstructure than castings of the same alloy. As a result, their mechanical properties are typically better than those of castings. While the fabrication and machining of components from various thermo-mechanically worked product forms is possible, the process is labor intensive and produces much scrap. For these reasons, it is quite expensive, and casting is a preferred process. Castings are sometimes hot isostatically pressed (HIP'd) to enhance properties.
The well known nickel base superalloy INCONEL.RTM. Alloy 718 has been used by the gas turbine engine industry for many years. INCONEL is a registered trademark of The International Nickel Company, Inc. Hereinafter, INCONEL Alloy 718 will be referred to as IN718. This alloy is described in Aerospace Materials Specifications (AMS) 5663 (wrought products) and AMS 5383 (cast products). According to AMS 5383, the composition range for IN718 is, by weight percent, 50-55 Ni, 17-21 Cr, 4.75-5.5 Cb+Ta, 2.8-3.3 Mo, 0-1 Co, 0.65-1.15 Ti, 0.4-0.8 Al, 0.0-1.75 Al+Ti, 0.0-0.35 Si, 0.0-0.006 B, 0.0-0.30 Cu, 0.0-0.015 S, 0.0-0.015 P, 0.0-0.35 Mn, 0.0-0.10 C, with the balance Fe. As shown in Table I, IN718 in wrought form has better mechanical properties than the alloy in cast+HIP form. In the Table, wrought IN718 specimens were processed into bars and forgings according to AMS 5663 requirements. Cast+HIP IN718 specimens were HIP'd at 2,175.degree. F. for 4 hours at 15,000 pounds per square inch (psi) in argon and then heat treated to optimize mechanical properties.
The desirability of casting large, complex IN718 components to near-net shape which require a minimum of post-casting processing has long been apparent. Such a capability would substantially decrease the ultimate cost of the component due to the elimination of forging, machining, and joining operations.
A development program was conducted to examine the possibility of casting IN718 into large structural components for turbomachinery such as gas turbine engines. After solving many casting related problems, it was noticed that porosity, segregation, and inclusions were still present in the castings to undesirable levels. Such defects are detrimental to mechanical properties, and must be eliminated if the use of large IN718 cast components is to become practicable. In order to reduce the porosity and segregation, the castings were given a hot isostatic pressing treatment, which was found to reduce the number of some of these defects. Following the HIP treatment, attempts were made to weld repair remaining casting defects; weld repair of such defects by e.g., gas tungsten arc or gas metal arc welding techniques is well known in the art. However, during the repair of these defects, difficulty was encountered. This difficulty was evidenced in the form of substantial outgassing and weld splatter which was generated during the repair process. Additionally, metallographic examination of the welds indicated an unacceptable and abnormal quantity of gas holes in the weld, the holes shown by arrows in FIG. 1; microcracks in the heat affected zone (HAZ) (shown by arrows in FIG. 2) were also detected. After a detailed investigation, it was found that the difficulties encountered during weld repair, and the gas holes in the weld were the result of entrapment of the high pressure HIP media (argon gas) during the HIP treatment in pores connected to the surface either directly or by way of grain boundaries. The gas entrapment apparently resulted when localized melting of the component occurred during the elevated temperature HIP treatment. Gas that had penetrated into the component by way of surface connected porosity or liquated grain boundaries was trapped as the locally melted material dissolved into the matrix by thermal homogenization during the HIP treatment, and as the component cooled to room temperature at the conclusion of the HIP treatment. Metallographic studies indicated an unusually large amount of the low melting Laves phase in the same areas that gas entrapment was found. In IN718, the Laves phase is believed to have the general formula (Ni, Fe, Cr, Mn, Si).sub.2 (Mo, Ti, Cb).
Laves phase was also found to be the primary cause of the observed HAZ microcracking, although it was determined that such cracking was independent of the entrapment of argon gas during the HIP treatment. These cracks are generally subsurface, and may significantly decrease the life of welded components; as a result, they are undesired. A detailed analysis of the relation between Laves phase and HAZ microcracking is presented in Vincent, "Precipitation Around Welds In the Nickel Base Superalloy Inconel 718", Acta Metallurgica, Vol. 33, No. 7 (1985) pp. 1205-1216.
It has been determined that cast IN718 which contains Laves phase may be heat treated so as to dissolve substantially all of the Laves phase prior to HIP processing. See the copending and commonly assigned application, PRE-HIP HEAT TREATMENT OF SUPERALLOY CASTINGS, U.S. Ser. No. 565,589. The heat treatment renders the alloy more easily weldable: due to the absence of Laves phase, gas entrapment during HIP is substantially eliminated. However, this heat treatment is time-consuming, and best avoided if possible.
In a program which led to the development of the alloys of the present invention, metallographic examination was conducted to determine if there was a relationship between the quantity of Laves phase precipitate which formed in cast IN718 and the specimen solidification rate. The term "solidification rate" is meant to describe the rate of cooling between the alloy's solidus and liquidus temperatures. This examination revealed that the amount of Laves phase precipitate in as-cast specimens increased with decreasing (i.e., slower) solidification rates. This may be better seen by reference to FIGS. 3, 4, and 5. FIG. 3 is a photomicrograph of an IN718 test specimen solidified at a rate of about 5.degree. F. per minute; it should be noted that at this relatively slow solidification rate, there is a substantial amount of Laves phase in the microstructure, in the form of an interconnected network of precipitate in interdendritic regions. FIG. 4 is a photomicrograph of an IN718 test specimen solidified at a rate of about 150.degree. F. per minute. At this relatively fast cooling rate, the amount of Laves phase is considerably decreased compared to FIG. 3. Also, the Laves phase is present as isolated pools of precipitate, as compared to the interconnected network of FIG. 3. It should be apparent that if the interconnected Laves network of FIG. 3 melts during HIP, a substantially greater amount of gaseous HIP media may become entrapped in the alloy as compared to the amount entrapped if the Laves phase in FIG. 4 melts. FIG. 5 shows that the amount of Laves phase precipitate in cast IN718 is inversely proportional to the solidification rate of the alloy, i.e., more Laves phase forms as the solidification rate decreases. In the Figure, "Area Percent Laves Phase" was determined by optical microscopy at a magnification of 100.times.. The specimens shown in FIGS. 3 and 4 were prepared using standard metallographic techniques. To highlight the Laves phase precipitate, the specimens were electrolytically etched with an aqueous solution containing 10% oxalic acid. In these photomicrographs, the Laves phase appears as the white phase while the dark phase surrounding the Laves is predominantly the gamma double prime phase, Ni.sub.3 Cb. The gamma double prime phase is the primary strengthening phase in IN718; as such, the alloy, as well as those compositionally similar to it, are referred to as gamma double prime strengthened alloys. The matrix phase in IN718 is a nickel solid solution, gamma. Dispersed within the gamma phase are carbides, which also appear white in the photomicrographs. micrographs.
Laboratory and metallographic analysis of the Laves phase in IN718 revealed that it had a melting point of about 2,100.degree.-2,125.degree. F. This is considerably less than the IN718 solidus and liquidus temperatures, which are about 2,325.degree. F. and 2,510.degree. F., respectively, when Laves phase is not present. It is also less than a commonly used HIP temperature of 2,175.degree. F., which accounts for the observed Laves phase melting during the HIP treatment, as discussed above. The Laves phase hardness was determined to be about 60 Rockwell C. Electron microprobe microanalysis of the Laves phase indicated that its composition was, on a weight percent basis, about 35-40 Ni, 25-30 Cb, 11-13 Fe, 11-13 Cr, 7-10 Mo, 1-2 Ti, 1 Si; this composition is in agreement with the composition reported in the above-mentioned articles by Vincent. U.S. Pat. No. 4,431,443 states, however, that in IN718, Laves phase is stoichiometrically written as Ni.sub.2 Cb, i.e., its composition is, by weight percent, 56 Ni-44 Cb.
In accordance with the trend shown in FIG. 5, it was found that in large, complex IN718 castings such as gas turbine engine diffuser cases, Laves phase was present in thick sections, and in other sections which due to inherent requirements of the casting operation (e.g., mold design, core placement, etc.) solidified at slow rates. For some currently used jet engines, as-cast diffuser cases may weigh up to about 1,000 pounds, and have section thicknesses which range between about 0.75 inch and 0.10 inch. In some thick sections, the solidification rate is estimated to be about 5.degree. F. per minute; in some thin sections, the solidification rate is estimated to be about 150.degree. F. per minute. Referring to FIG. 5, if IN718 is cast under these kinds of conditions, Laves phase will form in slowly solidifying areas. As discussed above, the presence of Laves phase renders IN718 unweldable, i.e., there is an unacceptable amount of outgassing and weld splatter generated, and microcracks in the HAZ are formed.
In a related program, it was determined that the tensile properties of cast+HIP IN718 were reduced by the presence of Laves phase in the microstructure, compared to specimens whose microstructure contained little or no Laves phase. See Table II, which presents data for cast+HIP IN718 specimens which had a considerable amount of Laves phase in the microstructure, similar to the amount present in the specimen shown in FIG. 3. Table II also presents data for cast+HIP IN718 specimens containing no Laves phase. These Laves free IN718 specimens were given a heat treatment prior to HIP processing which dissolved all of the Laves phase detectable at 100X resolution. This heat treatment caused no other detectable microstructural or metallurgical changes in the material. The HIP treatment for all specimens in the Table was 2,125.degree. F. for 3 hours at 15,000 psi. Subsequent to the HIP treatment, all specimens were given a stabilization heat treatment at 1,600.degree. F. for 10 hours, a solution heat treatment at 1,750.degree. F. for 1 hour and a precipitation heat treatment at 1,350.degree. F. for 8 hours, followed by a furnace cool at a rate of at least 100.degree. F. per hour to 1,225.degree. F., and holding at 1,225.degree. F. for 8 hours. As is seen in the Table, the presence of Laves phase causes a debit in properties at both test temperatures. Ductility (i.e., reduction in area and elongation) and stress rupture are significantly reduced.
The alloys of the present invention result from an extensive program to develop alloys which have properties comparable to similarly processed IN718, and which can be cast into large, complex, and near-net shapes, have a microstructure characterized by little or no Laves phase or entrapped gas in the cast+HIP condition, and which can be welded to repair as-cast defects such as porosity or inclusions without outgassing or the generation of weld splatter, and without forming weld cracks.